The present invention relates to methods and apparatus for controlling the dissolution of a gas in a liquid and more particularly methods and apparatus for automatically controlling the supply of a feed gas, the rate of dissolution and power required in accordance with the gas demand of the liquid.
In numerous instances, it is necessary to effect the dissolution of a gas in a liquid. Many times, the only constraint upon methods and apparatus for dissolving gases in a liquid is the electrical or other power required for generating mixing energy necessary to effect the required mass transfer. This is the case, for example, in the course of air aeration of mixed liquor in the secondary stage of an activated sludge process. However, in numerous other processes, the gas to be dissolved in a liquid is costly and therefore must be utilized efficiently to render such processes economical. For example, in the activated sludge waste treatment process, it has been found that secondary stage BOD reductions can be accomplished in a much shorter time and consequently the throughput of any system can be dramatically increased by utilizing a gas such as commercially pure oxygen in the treatment of mixed liquor. However, the requirement to conserve costly gases places an added constraint upon dissolution systems above and beyond the necessity to utilize electrical power as efficiently as possible.
In many processes, such as for example the activated sludge waste treatment process, both the demand of the liquid to be treated and the flow rate of such liquid tend to vary randomly and significantly over periods of time. Consequently, the loading, (which may be defined as the demand exhibited by a liquid for a particular gas) of mixed liquor in a secondary stage activated sludge process will vary widely at different times of the day. Consequently, in an "oxygen" wastewater treatment system, the oxygen flow to the particular dissolution device utilized must be carefully controlled in accordance with such variable loadings and flow rates. In a typical prior art system for dissolving oxygen in mixed liquor, as for example illustrated in U.S. Pat. No. 3,547,815, a simple control valve is utilized as a means for controlling the supply of feed oxygen to a dissolution device which is essentially comprised of a submerged sparger and rotating impeller in a covered aeration basin. Generally such impellers are driven at a predetermined rate to enable dissolution of oxygen at a rate sufficient to meet a particular O.sub.2 demand at the mixed liquor. However, in the event that the loading of mixed liquor to be oxygenated substantially decreases, excessive electrical energy is utilized to continue driving such impellers at the predetermined rate. Accordingly, such dissolution devices are exemplary of prior art systems which have not been fully effective in simultaneously controlling gas utilization and electrical power necessary to effect the desired mass transfer of gas into the liquid under treatment.
In a further dissolution system such as the apparatus described in U.S. Pat. No. 3,826,742 and which is assigned to the assignee of the present invention, a gas to be dissolved is injected into liquid flowing through an enclosure and as such liquid undergoes a gravitational fall, the aforementioned gas is dissolved therein. The height of the gravitational fall is detected and related to the consumption of gas by the liquid such that upon the fall height decreasing, a valve in the gas feed line is controlled to augment the gas supply and thereby re-establish a predetermined fall height. While this apparatus is particularly suitable for the treatment of liquids such as mixed liquor of a secondary stage activated sludge process, in an elongated enclosure or pipeline, this apparatus is not well suited toward improving operation of conventional open tank secondary stage aeration systems. Consequently, prior art gas dissolution methods and apparatus reflect a clear need for techniques for efficiently utilizing and consuming an oxygen enriched feed, while additionally controlling the rate of dissolution in accordance with the loading of a liquid and thus enable reductions in electrical power particularly upon occurrence of significant reductions in such loading.